In space, a constant stream of cosmic rays from outside the solar system raise an astronaut's lifetime odds of cancer and cataracts, and the specter of radiation sickness comes from exposure to solar storms. Among the many hazards of space travel, radiation risks have emerged as a top concern for burgeoning space travel plans for Mars, a roughly six-month trip contemplated both by NASA and private teams.

Based on measurements from the space agency's Curiosity rover which arrived on Mars last year, researchers led by Cary Zeitlin of the Southwest Research Institute in Boulder, Colo., report that astronauts would likely receive a significant fraction, about 15% to 20%, of their lifetime allowable radiation dose on a round-trip to Mars. "The best thing to do is to get there faster," Zeitlin says, in comments to the journal Science, which released the study.

In space, the rover was shielded by the spacecraft's aluminum skin and landing gear, providing roughly the amount of shielding that astronauts could expect on their way to Mars.

"By our best current estimates, the cancer risks would not be expected to be a 'show stopper,' but there is still a lot we don't know," says radiation expert Sally Amundson of the Columbia University Medical Center in New York, who was not part of the study. Radiation also poses risks of cataracts, neurological disorders, and heart and lung damage that is less well-understood than cancer risks.

Underlining space agency interest in a Mars trip, NASA Administrator Charles Bolden spoke Thursday at an Institute of Medicine panel meeting on the safety and ethics of long-duration space travel. President Obama's 2010 space exploration plan calls for a trip to a nearby asteroid and then Mars.

During its 253-day trip, the rover recorded five solar storms that greatly raised the radiation dose it measured in space. Astronauts walking on the surface of Mars would largely be unshielded from space radiation unlike the protection they might have here on Earth. That would further raise their radiation exposure and could put Mars visitors near the point of a 3% lifetime risk of cancer that NASA uses as a limit for each astronaut's career.

Because cosmic rays would require prohibitively heavy amounts of screening, Zeitlin suggests that efforts to understand and better predict solar storms might be most useful for warnings to astronauts. Some space planners have proposed adding a more heavily shielded refuge for Mars travelers to weather storms, similar to the way International Space Station astronauts move to more heavily shielded sections during some solar storms. Others have suggested that Mars visitors live in lava tube caves, whose openings have been seen from space, during their time on the Red Planet.

"There are a whole lot of risks in space, and there are many people who are willing to volunteer to face them," says University of Washington radiation safety expert Jeffrey Schwartz. He notes that exposure to acute radiation from solar storms poses the biggest risk to astronauts actually completing their missions, echoing Zeitlin. "Astronauts absolutely need to be informed of their radiation risk, that is a bottom line," Schwartz says. "We also have to look at acute effects, dangers to the eyes or neurological problems, that could threaten a mission."